Dual functional WO3/BiVO4 heterostructures for efficient photoelectrochemical water splitting and glycerol degradation

Dual functional heterojunctions of tungsten oxide and bismuth vanadate (WO3/BiVO4) photoanodes are developed and their applications in photoelectrochemical (PEC) water splitting and mineralization of glycerol are demonstrated. The thin-film WO3/BiVO4 photoelectrode was fabricated by a facile hydrothermal method. The morphology, chemical composition, crystalline structure, chemical state, and optical absorption properties of the WO3/BiVO4 photoelectrodes were characterized systematically. The WO3/BiVO4 photoelectrode exhibits a good distribution of elements and a well-crystalline monoclinic WO3 and monoclinic scheelite BiVO4. The light-absorption spectrum of the WO3/BiVO4 photoelectrodes reveals a broad absorption band in the visible light region with a maximum absorption of around 520 nm. The dual functional WO3/BiVO4 photoelectrodes achieved a high photocurrent density of 6.85 mA cm−2, which is 2.8 times higher than that of the pristine WO3 photoelectrode in the presence of a mixture of 0.5 M Na2SO4 and 0.5 M glycerol electrolyte under AM 1.5 G (100 mW cm−2) illumination. The superior PEC performance of the WO3/BiVO4 photoelectrode was attributed to the synergistic effects of the superior crystal structure, light absorption, and efficient charge separation. Simultaneously, glycerol plays an essential role in increasing the efficiency of hydrogen production by suppressing charge recombination in the water redox reaction. Moreover, the WO3/BiVO4 photoelectrode shows the total organic carbon (TOC) removal efficiency of glycerol at about 82% at 120 min. Notably, the WO3/BiVO4 photoelectrode can be a promising photoelectrode for simultaneous hydrogen production and mineralization of glycerol with a simple, economical, and environmentally friendly approach.


Introduction
Clean water and clean energy are two urgent global goals to achieve a better and more sustainable future for human society. Biodiesel is a renewable energy that holds various aspects of sustainability. 1 However, during biodiesel production glycerol by-product is generated in large amounts. Crude glycerol is generated between 60 and 70 wt% from the transesterication process. 2 Although, glycerol is used widely in various industries such as pharmaceutical, medicine, cosmetics, toiletries, food, and personal care products, the glycerol by-product from biofuel production is still in surplus. Therefore, the surplus crude glycerol has resulted in its price reduction by 60% and has been regarded as a waste stream that required proper disposal. 3,4 In addition, continuously increasing energy demand and fossil energy exhaustion have driven the energy price to rise rapidly. Furthermore, the combustion of fossil fuels releases carbon dioxide (CO 2 ), which is one of the greenhouse gases that primarily causes global warming. To overcome these issues, the next-generation energy source should be sustainable and clean. Hydrogen gas has been considered a high-efficiency, storable, transportable, renewable, and environmentally-friendly energy. 5 Several engineering strategies have been studies to sustainably produce H 2 for instance pyrolysis, 6 thermolysis, 7 electrolysis, [8][9][10] biophotolysis, 11 and photoelectrolysis. 12 Photoelectrolysis is a process of water splitting by the integration of solar energy and electric power also known as the photoelectrochemical (PEC) process. The PEC process provides a sustainable way to produce H 2 due to the most abundant of solar energy. The PEC water splitting has been extensively studied with the aim to improve the efficiency and stability. 13,14 PEC water splitting from a semiconducting TiO 2 photoanode under UV illumination was rst reported by Fujishima and Honda in 1972. 15 Semiconductor electrodes play an important role in PEC water splitting. The primary reasons are PEC water splitting process requires three fundamental steps, including light absorption, photoexcited charge separation and transportation, and photoexcited charge reaction. 13 During the past ve decades, earth-abundant and low-cost materials metal oxide semiconductors such as TiO 2 , [16][17][18] 25 and BiVO 4 26,27 have been extensively explored as photocatalyst materials in the PEC system. These studies reveal that metal oxide materials have great potential to be promising photoelectrode materials for PEC water splitting. However, using a single semiconductor photocatalyst still holds great challenges for obtaining high PEC performance. For instance, insufficient light absorption, 28 inefficient charge separation and charge transportation, 29 and photo-corrosion are the main factors that limit the PEC water splitting performance. 13,14,30 Fortunately, WO 3 and BiVO 4 are two of the most promising photoanode materials, due to their chemically stable, and narrow band gap (WO 3 E g = 2.5-2.7 eV, BiVO 4 E g = 2.4 eV) 20,26 Especially, heterostructure WO 3 /BiVO 4 has been reported to improve charge transfer and alleviate charge recombination. 29,[31][32][33] Herein, we developed dual-function WO 3 /BiVO 4 photoelectrodes for both converting energy from renewable energy sources and simultaneously removal of glycerol pollutant. The WO 3 /BiVO 4 photoelectrode is composed of a superior structure of the monoclinic structure of WO 3 and monoclinic scheelite structure of BiVO 4 that provided good electron transport and photocatalytic activity, respectively.
In addition, the constructed type-II heterostructure of WO 3 / BiVO 4 effectively reduced charge recombination by facilitating electron separation between WO 3 and BiVO 4 . Furthermore, the oxygen vacancy in the WO 3 /BiVO 4 further improves the charge separation of WO 3 /BiVO 4 . The photoelectrochemical performance showed that the photocurrent density of the WO 3 /BiVO 4 photoelectrode was 5.12 mA cm −2 at 1.23 V vs. RHE under simulated AM 1.5G illumination, which was 2 times higher than that of the pristine WO 3 photoelectrode in the presence of 0.5 M Na 2 SO 4 electrolyte. The photocurrent density of the WO 3 /BiVO 4 photoelectrode was further improved to 6.85 mA cm −2 and the TOC reached 80% in the presence of a mixture of 0.5 M Na 2 SO 4 and 0.5 M glycerol electrolyte. These results highlighted a simple and economical approach to fabricating WO 3 /BiVO 4 photoelectrodes that exhibit a considerable performance for dual PEC water splitting and contaminant degradation.

Characterization of WO 3 and WO 3 /BiVO 4 photoelectrodes
The morphology of the nanostructured photoelectrodes was examined by SEM as shown in Fig. 1(a-d). Fig. 1(a) shows that the FTO substrate is densely and uniformly covered by vertically-aligned two-dimensional (2D) WO 3 nanoplates. Fig. 1(b) is the higher magnication SEM image showing WO 3 2D nanoplates with smooth surface and the thickness estimated to be ∼100 nm. Fig. 1(c) and (d) show the WO 3 nanoparticles which are deposited by BiVO 4 , which is so called WO 3 /BiVO 4 thereaer. Obviously, WO 3 /BiVO 4 exhibits aligned structure, rougher surface, and much thicker than that of the bare WO 3 , indicating successful deposition of BiVO 4 .
planes of the monoclinic scheelite structure of BiVO 4 (JCPDS No. 14-0688), respectively. 34 In addition, no obvious diffraction peaks of WO 3 in the heterostructure WO 3 /BiVO 4 , and no other peaks are observed, suggesting that relatively thick layer of BiVO 4 deposited on the WO 3 surface. In addition, both monoclinic structure of WO 3 and monoclinic scheelite structure of BiVO 4 have been reported to have superior properties, where the monoclinic of WO 3 phase was reported to have faster electron transport than orthorhombic phase. 35 Moreover, the monoclinic scheelite structure of BiVO 4 was reported as the most photocatalytic active phase among the other two crystal structures, tetragonal scheelite structure, and tetragonal zircon structure. 36 The WO 3 and WO 3 /BiVO 4 photoelectrodes were analyzed by FTIR to investigate the presence of functional groups, as shown in Fig. 2(b). The peaks at 3500 and 1635 cm −1 can be ascribed to the stretching vibration and the bending mode of the hydroxyl group (-OH), respectively. The hydroxyl group found was due to the atmospheric humidity adsorbed onto the surface. 31 In addition, the hydroxyl group provides a benecial effect for trapping charge carriers to create reactive hydroxyl radical (OHc) that effectively oxidizes organic molecules. 37 The crystal structure of WO 3 and WO 3 /BiVO 4 was further investigated by Raman spectroscopy. Fig. 2 31 These results indicate that WO 3 /BiVO 4 is successfully fabricated. In addition, it is obvious that the most intense peak of the WO 3 and WO 3 /BiVO 4 shis from 798 to 813 cm −1 , suggesting non-stoichiometric of WO 3 /BiVO 4 due to oxygen vacancies.
To conrm the presence of oxygen vacancies, the chemical state of the WO 3 /BiVO 4 photoelectrodes was analyzed by XPS. Fig. 3(a-d) show high resolution spectra for the W 4f, O 1s, Bi 4f, and V 2p, respectively. As shown in Fig. 3(a), the W 4f spectra reveals four curves aer convolution using Gaussian   Fig. 3(d) shows V 2p spectra with two peak located at 516.9 and 522.7 eV, which can be assigned to V 2p 3/2 and V 2p 1/2 of V 5+ state, respectively. 31 The light absorption spectra of the photoelectrodes were analyzed using the UV-Vis DRS technique to reveal the lightharvesting property as shown in Fig. 4(a). The absorption edge of the WO 3 nanoplates is around 470 nm, while the absorption range was extended to around 520 nm for WO 3 /BiVO 4 . The band gap of the WO 3 and WO 3 /BiVO 4 are 2.64 and 2.38 eV, respectively as shown in Fig. 4(b). The WO 3 /BiVO 4 samples showed good absorption in the visible light region, which can facilitate the enhancement of PEC properties under visible light irradiation. 33,41 Fig. 5(a) and (b) show TEM images and the corresponding EDS elemental mapping of WO 3 and WO 3 /BiVO 4 photoelectrodes. As shown in Fig. 5(a), W and O elements are uniformly distributed on the nanoplates without other impurities.
The weight percentage of W and O elements were estimated at 80.7 wt% and 19.3 wt%, respectively. Fig. 5(b) shows the elemental mapping of WO 3 /BiVO 4 photoelectrode. The result shows that W and O are uniformly distributed throughout the whole particle, while Bi and V also presented in the elemental maps, indicating the existence of BiVO 4 in the WO 3 /BiVO 4 composite. The weight percentages of W, O, Bi, and V elements were estimated at 59.3 wt%, 17.5 wt%, 19.3 wt%, and 3.8 wt%, respectively. Furthermore, the high-resolution transmission electron microscope (HRTEM) images of the selected WO 3 and WO 3 /BiVO 4 nanoplates were conducted as shown in Fig. 5(c) and (d). Fig. 5(c) shows the measured lattice fringes with interface spacings of 0.36 nm and 0.37 nm which corresponded to the (200) and (020) crystallographic planes of monoclinic WO 3 crystal phase, respectively, suggesting that WO 3 nanoplates have highly crystallized monoclinic structure. Fig. 5(d) shows the HRTEM of the composite WO 3 /BiVO 4. It reveals that the lattice fringe with interface spacings of 0.36 nm and 0.37 nm, which belong to the WO 3 monoclinic structure and additional lattice fringe with interface spacings of 0.30 and 0.47 nm that can be

Photoelectrochemical performance
The PEC performance of WO 3 and WO 3 /BiVO 4 photoelectrodes was evaluated by measuring the transient photocurrent response (I-t curve) in the presence of 0.5 M Na 2 SO 4 and an equal concentration of Na 2 SO 4 and glycerol at 0.5 M electrolyte at the applied potential of 1.23 V (vs. RHE) under simulated AM 1.5 G illumination as shown in Fig. 6(a and b). Fig. 6(a) shows the photocurrent density of WO 3 and WO 3 /BiVO 4 photoelectrodes in presence of 0.5 M Na 2 SO 4 electrolyte. The pristine WO 3 photoelectrode photocurrent density is 2.40 mA cm −2 which is comparable to other reported PEC systems of WO 3 photoelectrode. 29,31,32 Interestingly, the photocurrent density of the WO 3 /BiVO 4 photoelectrode increases to 5.12 mA cm −2 , approximately 2 times higher than that of the pristine WO 3 photoelectrode. The improved photocurrent density of WO 3 / BiVO 4 photoelectrode could be attributed to a synergistic effect of the crystal structure, light absorption, and charge separation. First, the monoclinic structure of WO 3 and the monoclinic scheelite structure of BiVO 4 provide fast electron transport and  superior photocatalytic activity, respectively. Second, the light absorption of WO 3 /BiVO 4 samples as shown in Fig. 4(a) that the absorption range was extended to around 520 nm compared to WO 3 nanoplates that have the main absorption edge around 470 nm. Thus, good absorption in the visible light region can facilitate the enhancement of PEC properties under visible light irradiation. 31 Third, the WO 3 /BiVO 4 forms a type-II heterostructure which effectively reduces charge recombination by facilitating electron separation between WO 3 and BiVO 4 . 32,38 In addition, the oxygen vacancy presence in WO 3 /BiVO 4 from the XPS analysis as shown in Fig. 3(b) can increase the driving force for charge separation of WO 3 /BiVO 4 . 31,38 To further improve the performance and additional functionality of PEC for H 2 production and waste degradation, glycerol was added to the NaSO 4 electrolyte to generate a bifunctional PEC system. It has been reported that small organic molecules such as glycerol waste can act as a sacricial agent and electron donor, promoting the photocatalytic water splitting performance. 42,43 As shown in Fig. 6(b), the photocurrent density of WO 3 and WO 3 /BiVO 4 photoelectrodes in presence of a mixture of 0.5 M Na 2 SO 4 and 0.5 M glycerol electrolyte is substantially increased. The photocurrent response of bare WO 3 photoelectrode is 3.41 mA cm −2 and increases to 6.85 mA cm −2 for WO 3 /BiVO 4 photoelectrodes. Interestingly, the photocurrent response of WO 3 / BiVO 4 photoelectrodes in the presence of Na 2 SO 4 and glycerol in an electrolyte compared to that of WO 3 photoelectrode in the presence of only NaSO 4 was increased by 2.8 times. The superior performance of WO 3 /BiVO 4 photoelectrodes is due to the abovementioned reasons and possibly due to the hydroxy group, which provides trapping for photoexcited holes and creates reactive hydroxyl radical (OHc) that effectively oxidize organic molecule, 37 from FTIR analysis as shown in Fig. 2(b). In addition, Fig. 6(c) shows the TOC removal by WO 3 and WO 3 /BiVO 4 photoelectrodes. This can be seen that the TOC removal of WO 3 / BiVO 4 photoelectrodes substantially increased and reached 82%, while WO 3 photoelectrodes gradually increased and reached 40%. Thus, the WO 3 /BiVO 4 photoelectrodes provide efficient charge transfer and separation, which in turn contribute to both effective PEC water splitting and contaminant degradation.
Based on the above discussions, the mechanism of the dual functional PEC system of photoelectrode is illustrated in Fig. 7, the conduction band (CB) of BiVO 4 is close to the hydrogenreduction potential and the photo-excited electrons can thermodynamically transfer from the high CB energy level of BiVO 4 to the more positive CB of WO 3 . Holes in the valence band (VB) of WO 3 can move spontaneously to the VB of BiVO 4 for water oxidation. These bandgap differences between WO 3 and BiVO 4 also enhance the charge separation and reduce the bulk's charge recombination rate. As a result, the WO 3 /BiVO 4 photoelectrodes exhibit much better PEC performance than that of the bare WO 3 photoelectrode. Fig. 8 shows the preparation of WO 3 and WO 3 /BiVO 4 photoanodes by a hydrothermal method. Firstly, 0.1 g of sodium tungstate dihydrate (Na 2 WO 4 $2H 2 O) was dissolved in 10 mL of Milli-Q water. Then, 3 mL of 2 M of hydrochloric acid (HCl) was added dropwise in the above solution under constant stirring at room temperature for 15 min. Subsequently, 0.2 g of citric acid (C 6 H 8 O 7 ) and 15 mL of Milli-Q water were added into the mixture with continual stirring for 20 min. Aerward, 30 mL of the prepared precursor solution was then transferred to a 50 mL Teon-lined stainless-steel autoclave. Before immersing FTO glass into the autoclave, FTO glass substrate was cleaned by ultrasonic treatment using acetone, ethanol, and isopropanol (each for 15 min), followed by drying in a nitrogen stream. Aer the FTO glass was immersed in an autoclave with the FTO side leaned down against the wall, the autoclave was sealed and kept at 120°C for 8 h. Aer the hydrothermal process, the samples were rinsed with Milli-Q water, and dried in a nitrogen stream. Finally, WO 3 photoelectrodes were obtained aer annealing in air at 500°C for 1 h. For WO 3 /BiVO 4 photoelectrodes, the precursor of BiVO 4 was prepared by dissolving 1.4 g of bismuth(III) nitrate pentahydrate (Bi(NO 3 ) 3 $5H 2 O), 0.8 g of vanadium acetylacetonate (C 10 H 14 O 5 V), 1 mL of acetic (CH 3 COOH), and 19 mL of acetylacetone (C 5 H 8 O 2 ) with sonication until the solution's color changed to dark green. Then, the prepared WO 3 photoelectrode was immersed into the BiVO 4 precursor solution for 30 min. Aerward, the above sample was annealed in air at 450°C for 3 h to obtain the WO 3 /BiVO 4 photoelectrodes.

Characterization
The surface morphology of the as-prepared photoelectrodes were examined using a eld emission scanning electron microscope (FE-SEM, JSM-7610FPlus, JEOL, Tokyo, Japan). Transmission electron microscope equipped with an energydispersive X-ray spectroscope (TEM/EDX) and high-resolution TEM (HRTEM) analyses were conducted by JEOL2100 Plus, operated at 200 keV. The crystalline phases of the photoanodes were characterized by X-ray diffraction (XRD; Bruker, D8 Discover, Germany) using the Cu Ka radiation in a 2q range of 20°-80°. The functional group of the photoelectrodes were analyzed by FTIR spectroscopy and recorded over a region of 4000-650 cm −1 (Nicolet 6700, Thermo Scientic, USA). Raman spectra were recorded over a spectral range of 200-1000 cm −1 and collected on a Horiba XploRA PLUS instrument, Japan. The elemental states were analyzed by X-ray photon spectroscopy (XPS, Kratos AXIS Ultra DLD). The light absorption spectra were investigated by a UV-Vis spectrophotometer (JASCO V-630).

Photoelectrochemical measurement
The dual functional PEC measurements were performed in an H-cell type PEC cell with a quartz window and tested on a CHI 660D electrochemical workstation. The prepared photoelectrodes, a Pt wire (1 mm diameter), and a Ag/AgCl electrode served as the working electrode, counter electrode, and reference electrode, respectively. The illumination area was set by an aperture diameter of 1 cm. A xenon lamp (100 W, Newport LCS-100) was used to simulate sunlight and the photocurrent densities were measured under solar AM 1.5 G. The PEC behaviour of the electrodes was characterized with degradation of 100 mL of a mixture of 0.5 M glycerol and 0.5 M Na 2 SO 4 Fig. 7 Schematic visualization of the bifunctional PEC system (hydrogen generation and glycerol degradation) using H-cell type PEC reactor. solution at room temperature. All solutions were prepared from Milli-Q water. The solutions were purged with nitrogen gas for 30 min prior to PEC measurement. To evaluate the degradation of glycerol, 5 mL of the treated solution was sampling every 30 min and analyzed by a Shimadzu TOC-V CPN Total Organic Carbon Analyzer. A 150 W Xe lamp light source with the intensity of the simulated 1-Sun solar illumination condition (AM 1.5, 100 mW cm −2 ) illuminated at the immersed photoelectrode in the solution. Potentials versus RHE were calculated using the Nernst equation E RHE = E Ag/AgCl + 0.0591(pH) + 0.1976 V.

Conclusion
In summary, a simple hydrothermal technique is developed for fabricating the heterojunction WO 3 /BiVO 4 photoanodes. The WO 3 /BiVO 4 photoanodes take advantage of both the superior electron transport of the monoclinic structure of WO 3 and the good photocatalytic activity of the monoclinic scheelite structure of BiVO 4 . The photocatalytic activity of the pristine WO 3 and WO 3 /BiVO 4 photoanodes are 2.40 mA cm −2 and 5.12 mA cm −2 in the presence of 0.5 M Na 2 SO 4 electrolyte, respectively. The improved photocurrent density of WO 3 /BiVO 4 photoelectrode could be attributed to a synergistic effect of the superior crystal structure, type-II heterostructure, and the presence of oxygen vacancies. Furthermore, the photocurrent density of WO 3 /BiVO 4 photoelectrodes was improved to 6.85 mA cm −2 and the TOC removal efficiency reached about 82%, in the presence of a mixture of 0.5 M Na 2 SO 4 and 0.5 M glycerol electrolyte. The photocurrent density of the WO 3 /BiVO 4 photoelectrodes is about 2.8 times higher than that of the pristine WO 3 in the presence of 0.5 M Na 2 SO 4 electrolyte. The considerable enhancement is due to the afore-mentioned synergistic effects and the hydroxy group that provides trapping for photoexcited holes and creates reactive hydroxyl radicals (OHc) that effectively oxidizes organic molecules.

Conflicts of interest
There are no conicts to declare.